1. Field of Invention
This invention relates to a photoacoustic instrument or sensor for measuring particulate emissions from vehicles and other combustors. It can be placed at a roadside location. The instrument is also used as a primary method for quantitatively measuring aerosol light absorption in the atmosphere.
2. Description of Related Art
Internal combustion engines used to power vehicles generate both gaseous and particulate emissions as they travel on roads and expressways. The gaseous emissions have been extensively sampled and tested. Particulate emissions have proven to be more difficult to sample and test at a roadside location. Photo-acoustic gas sensors have been used to detect concentrations of gases such as carbon monoxide and other hydrocarbons. Photo-acoustic gas sensors generate an acoustic pressure wave when the gas is irradiated with a modulated light source in a sample chamber. The radiation absorbed by the gas results in pressure variations in a given volume of gas. The pressure variation is proportional to the amount of energy absorbed. A microphone can detect the pressure wave. The magnitude of the wave is proportional to the concentration of the gas. The acoustic pressure wave arises as the gas absorbs the optical radiation and is heated. Periodic thermal expansion and pressure fluctuations result, corresponding to the modulation of the optical radiation. Measurement of the acoustic pressure then permits inferring the gas concentration. Different gases are characterized by the use of light waves of different wavelengths corresponding to the absorption wavelength of the gas being tested.
Photoacoustic sensors have a high degree of measurement sensitivity and have to be carefully designed in order to prevent external noise from generating erroneous results. One problem that occurs with photoacoustic sensors is that they can receive acoustic signals (noise) from outside the sample chamber. This noise enters the sensor through the same entrance as the sample gas. The external acoustic noise causes false readings.
Another problem with photoacoustic sensors is that they can drift out of calibration due to changing pressure on the microphone. Typical microphones have an electrically conducting membrane and a fixed back plate. If air is moving in the test chamber, the pressure on the membrane can be different than the pressure on the back plate causing the sensor to go out of calibration.
Another problem with photoacoustic sensors is that the light source has to be aligned with the sample chamber to obtain accurate readings. If the light source is an infrared laser, it cannot be visually aligned. For safety reasons, it is desirable to enclose the laser so that the laser cannot accidentally contact an eye.
A current unmet need exists for a roadside particulate emission sensor that is not influenced by external noise, that is readily calibrated, that stays in calibration and that has an easily alignable light source.
1. Advantages of the Invention
An advantage of the present invention is that it provides a sensor for detecting particulate emissions at a roadside location.
Another advantage of the present invention is that it provides a photoacoustic instrument that can measure black carbon particles.
A further advantage of the present invention is that it provides a photoacoustic sensor that uses cameras to align a laser beam.
An additional advantage of the present invention is that it provides a photoacoustic sensor that prevents external noise from generating erroneous results.
Yet another advantage of the present invention is that it provides a photoacoustic sensor that prevents calibration drift due to changing pressure on a microphone.
A further advantage of the present invention is that it provides a photoacoustic sensor that is compact and easily transported.
A further advantage of the present invention is that it provides a photoacoustic sensor that is easily calibrated.
These and other advantages of the present invention may be realized by reference to the remaining portions of the specification, claims, and abstract.
2. Brief Description of the Invention
The present invention comprises a photoacoustic sensor for measuring light absorbing particles in a gas. The most common particles sampled are black carbon or soot, though the choice of light wavelength allows other particles to be sampled. The photoacoustic sensor comprises an acoustic waveguide and a modulated source of light located in proximity to the waveguide. The modulated light irradiates the particle-laden gas in the waveguide. A microphone is attached to the waveguide. The microphone detects an acoustic signal generated by absorption of the light by the particles in the gas. The acoustic signal is proportional to the mass concentration of particles in the gas. A pump is mounted to the waveguide. The pump pulls the gas through the waveguide. A piezoelectric calibrator is mounted perpendicular to the waveguide. The piezoelectric calibrator provides a known acoustic signal for calibrating the sensor. A critical orifice is mounted between the pump and the waveguide. The critical orifice prevents noise generated by the pump from entering the waveguide. Helmholtz resonators are mounted to the sample inlet of the waveguide. The [helmholtz]Helmholtz resonator prevents unwanted noise frequencies from entering the waveguide. A pressure equalizer is mounted between the critical orifice and the microphone. The pressure equalizer equalizes the pressure in proximity of the microphone.
The above description sets forth, rather broadly, the more important features of the present invention so that the detailed description of the preferred embodiment that follows may be better understood and contributions of the present invention to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and will form the subject matter of claims. In this respect, before explaining at least one preferred embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of the construction and to the arrangement of the components set forth in the following description or as illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.